1. Field of the Invention
This invention relates to a method of forming two or more Bragg gratings in an optical fibre waveguide.
2. Related Art
In this specification the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region, together with those parts of the infra-red and ultraviolet (UV) regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
There is considerable interest in exploiting photosensitivity in germanosilicate optical fibres for application in the areas of optical communications and sensors. The first reported permanent optically-induced changes of the refractive index of optical fibres was by K. O. Hill, Y. Fujii, D. C. Johnson and B. S. Kawasaki, "Photosensitivity in Optical Fibre Waveguides: Application to Reflection Filter Fabrication" Appl. Phys. Lett, 32, 647 (1978). In their experiment, coherent radiation at 514.5 nm, reflected from the fibre ends, generated a standing wave in the fibre which induced a periodic refractive index change along its length. This formed a high reflectivity Bragg grating in the fibre which peaked at the wavelength of the incident beam. Since then, numerous studies into the grating growth mechanism and photosensitive fibres have been carried out--see for example D. K. W. Lam, B. K. Garside, "Characterization of Single-Mode Optical Fibre Filters" Appl. Phys, Lett, 20, 440 (1981) and J. Stone, J. Appl. Phys., 62, 4371 (1987). However, the mechanism which results in the perturbation to the refractive index of the fibre core is not fully understood. The spectral region where the fiber is photosensitive has been found to range from the UV to around 700 nm.
The potential applications of fibre gratings are numerous. For example, in telecommunications applications, tunable integrated fibre gratings, externally written with a UV laser, may be used for spectral control of fibre lasers. B. S. Kawasaki, H. O. Hill, D. C. Johnson and Y. Fujii, in an article entitled "Narrow-band Bragg reflectors in optical fibres", Optics Letters Vol 3 No. 2 August 1978, pp 66-68, note that an important property of the grating formation process is the extent to which the filter response can be tailored. For example, one method of forming a complex filter is to superimpose two or more simple band-stop characteristics in the same fibre by illuminating the fibre with different wavelengths of light either simultaneously or consecutively.
Another known method of forming the Bragg gratings is by side-writing the gratings by interfering two coherent radiation beams at an appropriate angle. The pitch of the grating is determined by the angle of intersection of the two beams, so different grating pitches can be formed by adjusting this angle.